CI: use macos-15 to build the binaries

[ThomasWaldmann]

Github EOLed the macOS 13 Intel runner and only
supports Intel on macOS 15 from now on.

Let's also use macOS 15 for the Apple Silicon build.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.59%. Comparing base (1298840) to head (af232a5).
⚠️ Report is 3 commits behind head on 1.4-maint.

Additional details and impacted files

@@            Coverage Diff             @@
##           1.4-maint    #9228   +/-   ##
==========================================
  Coverage      80.59%   80.59%           
==========================================
  Files             38       38           
  Lines          11256    11256           
  Branches        1771     1771           
==========================================
  Hits            9072     9072           
  Misses          1615     1615           
  Partials         569      569           

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